Control system



Aug. 19, 1952 M. A. EDWARDS ETAL CONTROL SYSTEM Filed Jan. 22. 1949 Inventors; Martin 'A. Edwards, Robert A. Brand, y V7 The- Attorney.

Patented Aug. 19, 1952 I CONTROL SYSTEM Martin A. Edwards and Robert A.'Bra'nd, Schenectady, N. Y., assignors to General Electric Company, a corporation of NewYork Application January 22, 1949, Serial No. 72,214

11 Claims. (01. 318-154) Thisinvention relates to control systems, and

moreparticularly to systems for controlling the operation of electric motors and it has foran object the provision of a safe, reliable'and improved control system of this character.

More particularly, the invention relates to electric motor control systems for oil Well drilling rigs and the like in which the motor is used to supply power to hoist the drill stem out of the drill hole. It'is an object of the invention to reduce to a minimum the time required for such a hoisting operation. In this type of drive, the load is subject to great variation. At the time the hoisting operation starts with the entire drill stem or pipe in the hole, the full weight of the drill stem must be raised. At the end of the hoisting operation, the load approaches zero. The drill bit, which is attached to the bottom of the long drill pipe, has to be changed periodicallyin some cases, every few hours, and it is necessary to withdraw the entire length of drill pipe each time it is necessary to change the drill bit. The drill pipe is withdrawn from the hole by raising it a'short distance and removing a section of it, usually-90 feet in length, from the top of the drill pipe. This operation is repeated and sections areremoved from the top until the drill pipe is completely out of the hole. Since somewells are drilled to great depths, some as much as 20,000 feet,.it is very important that the hoisting'cycle be held to the minimum possible time. 1

Another problem connected with the drilling of oil wells and other wells where rotary drilling rigs are used is the holding of proper pressure on the drill bit during the actual drilling operation audit is another object of our invention to provide means for holding this pressure constant. When drilling a deep well, it is necessary to support at least part of the weight of the drill stem in order to reduce the pressure on the drill bit. If theentire weight of the drill stem is allowed to rest on the drill bit, the rate of progress of the hole;may be so great that the drilling chips are not removed from the hole, or it may result in the hole-being drilled crooked, or it might even result in the crushing of the drill bit in the case of a very long and heavy length of drill pipe.

Another object of our invention is to limit the speed of travel of the drill stem and its attached drill bit to a safe value inboth the hoisting direction andwhen drilling.

In carrying out our invention in one form, we provide a direct current motor electrically coupled to a direct current generator which is driven by a suitable prime mover. The motor is connected to the drill pipehoisting mechanism through a gear speed reducer. The motor acts'as a motor in the hoistin direction and as both a drag generator and motor in the loweringor drilling direction. The excitation of both generator and motor is variable and under the control of cross armature reaction excited dynamo-electric machines. f

For a better and more complete understanding of our invention, reference should be had to the following description and to the accompanying drawing, Fig. 1 of which is a simple diagrammatical illustration of one embodiment of our invention, while Fig. 2 illustrates a modification thereof.

Referring to Fig. l of the drawing, a direct current electric motor I is shown connected to' a direct current electric generator 2. The gener ator 2 is driven by a suitable prime mover 3, such as a diesel or steam engine. The motor I is connected through speed reducing gears to hoist drum 5. On drum 5 is wound a cable 6 which passes around multi-groove sheaves I and 8 and raises and lowers a drill stem 9 and a drill bit [0 attached to the bottom thereof, as cable 6 is wound 'or unwound from drum 5. One end of cable 6 is anchored at a suitable point ll. 1

The excitation voltages of motor I and generator 2 are controlled by special cros armature reaction excited dynamoelectric machines or 'amplidynes l2 and I3. These dynamoelectric ma? chines differ from a conventinal direct current machine in that they have a pair of auxiliary brushes l2a and 13a arranged on an axis that'is normally displaced substantially electrical degrees from the axis of the main load brushes I21) and BE). These auxiliary brushes I2a andl3a are substantially short-circuited by an external conductor. Control field windings [2c and lite and. 13d are arranged onthe axis of the main load brushes and provide a relatively small number of. ampere turns. However, the relatively small voltage induced in the armature between the auxiliary brushes by means of thecontrol flux produced by these control fields produces a very large current in the short-circuit and this short circuit current gives rise to a very large cross armature flux in line with the short-circuited brushes and, therefore, across, or at an angle, to the ordinary load current, armature reaction which is normally substantially in line with the main load brushes IZb and I311. It is this cross armature reaction flux which provides the main operating flux or excitation for dynamoelectric machines l2 and I3. By reason of the low inductance and low resistance of the circuit including the auxiliary brushes, a very small change in the control flux will produce a very large and very rapid change in the voltage between the main brushes [2b and l3b.

In order to operate the drilling apparatus in the hoist direction, solenoid I4 is tie-energized to close switches l5'and l6 and move switch I! to the lower position as shown in Fig. 1 on the accompanying drawing. Closing switches l5 and It causes a control or signal voltage to be supplied by resistor 20 to the energizing circuit of control field I20 of machine [2. age is the voltage drop across 'resistor 2fl caused by the current circulating in the series loop circuit l8 which is shown on the accompanying drawings connecting the armatures of motor I and generator 2.

Control field winding [20 is energized by being connected across a constant potential source of direct current from lines 2| and 22 through a manually variable resistor 23. The control or signal voltage fromresistor 20 is connected in serieswith control field winding l2c across this source of 'electrical'energy. The polarity of the supply and reference voltage derived from the constant potential source, and the polarity of the signal voltage from resistor 20 are additive so that an increase in the signal voltage causes an increased current to flow in control field winding l2c.

A small increase in the current in winding 120 causes a very large increase in the voltage between main brushes l2b on machine 12 with a resultant large increase in current in excitation field coil '24 of the main hoist motor I. The result is that the excitation of motor I increases proportionally as the armature current in'series loop 1'8 between motor I and generator 2 increases. In a like manner, the excitation of motor l decreases proportionally as the'armature current in the loop decreases.

By controlling the excitation of'motor l in this manner durin the hoisting operation, motor I is made to deliver approximately a constant horse- 'power output. When the load is'heavy, the motor runs slowly, but as soon as the load becomes lighter, the speed of the motor increases andthe motor continues to'deliver its maximum horsepower. In this way, the accelerating time and hoisting time are reduced to a minimum. When switch IT is in its lower position, as illustrated on the accompanying drawing, excitation field winding I30 of cross armature reaction excited dynamoelectric machine i3 is supplied with current from constant potential lines 2| and 22 through a manually variable resistor 25. This resistor is set to provide the desired'excitation for machine 13 in conformance with the capacity of the'apparatus and is normally left unchanged during the hoisting operation. Machine 13 supplies excitation voltage for field coil 26 of main generator 2.

The'excitation of generator 2' remains constant during the hoisting operation unlessthe speed of motorl exceeds a predetermined maximum. A tachometer generator 21 which is directly connected to the shaft of motor I, supplies a voltage which is proportional to the speed of -motor I. When motor I is operating in the hoisting direc-- tion, the signal voltage'generated by tachometer 21 is compared with'a reference voltage of like polarity derived from constant potential buses 2i and2 2 through a manually variable resistor 2B.

The signal volt- However, a rectifier 28a, which may be of the selenium type, in this circuit permits the flow of current in one direction only, this being when the voltage generated by tachometer 21 exceeds the reference voltage. When the latter is the greater, no current flows in either direction through winding Hid which is also in this circuit. If the voltagegenerated by tachometer 21 during the hoisting cycle exceeds a predetermined reference value, resulting from the manual adjustment of resistor 28, current flows through control field winding. l3d to counteract the effect of field I3c on the output of cross armature reaction excited dynamoelectric; machine l3. Field [301 is connected with opposite magnetic polarity to field thus, when current flows through field winding 13d it counteracts the magnetic effect of field [3c and the voltage across main brushes I3b of machine 13 is reduced. .This, in turn, reduces the excitation current flowing through field 26 of generator 2, thus reducing the current flowing in the series loop between motor I andgenerator Z. This reduces the torque 'exertedby motor 1 and its speed is reducedto a safe-value.

To operate the apparatus to control the weight on; the drill bit during the drilling operation, solenoid I4 is energized to open SWitChGSl-ial'ld I 6 and raise switch H tothe upper position. 'This causes motor 'I toact primarily as a drag generator while generator 2 acts as a motor and engine 3 as a power absorption device. It is assumed for purposes of this description that engine3 can-absorb the amount of-power which is returned'to it, either to make up its own'losses or to be passed on to another load (not shown) being. supplied by the engine. In-a case-where the engine cannot absorb sufilclent' power for anyreason, a power dissipating-resistormay be-inserted in series loop circuit l8 between motor-l and generatorz-to absorb excess power.

In order to maintain a constant predetermined pressure on drill bit Ill, machine I is made'to-exert a constant torque ongear speed reducer-4 and drumE, thus providingaconstant tension on rope 6 and through it a predeterminedconstan-t tension on drill stem 9. The tension on drill stem 9 is the difierence between-the total weight of drill stem9- and the drilling-pressure that 'itis desired to maintain on drillbit 1-D. Therefore, if the friction of the speed reducing gears and the rope and pulley system is ignored theodesired drilling pressure may be maintained on drill 'bit [0 by causing machine -I- to exert aconstant torque-omits output shaft. Constant torque is achieved on motor I- by providing it with constant excitation and then'holding its armature current constant regardless of changes in'speed or direction of rotation of motor I. a

To maintain the armaturelcurrent' inthe series loop circuit-between dynamo l and dynamo 2-=at a constantvalue, a signal or control voltageis derived'from-a resistor 29. This-voltage, which is proportional to the current flowing in series loop [8, is compared with a predetermined reference voltage derived from constant -potential lines 2| and 22 tlirough manually variable resistor 25. The polarities of the two voltages are additive so that an increase in the voltage drop across resistor 29 causes an increase in the-currentfiowing in control'fieldwinding [3c of machine [3. This increase in excitation is greatly amplified by machine It and the resultant-"amplified signal voltage istransmitted to main field winding-26 of generator 2. The increase "inexcitation or dynamo 2, which is -'oper-ating--now as =a"motor,

causes'the current in the armature series loop betweenmachine I and machine 2 to decrease until the voltage drop across resistor 29 is reduced to the desired value, returning the control and ex:- citation circuits of machine I 3 and dynamo 2 to a balanced condition. For a decrease in the current flowing in the series loop circuit between dynamo I and dynamo 2, the apparatus functions in the opposite manner to increase the series loop current and return it to the predetermined value.

Another method of maintaining the current in the seriesloop I8 at a value which will maintain the desired drilling pressure on drill bit I0 is by the use of a. strain gage in the dead side of rope 6, as illustrated in Fig. 2 on theaccompanying drawing. It will be understood that this eliminates the efi'ect of the friction in gear speed reducer 4 and most of the friction in the drum and in the remainder of the rope and pulley system and provides a signal producing arrangement which is more nearly directly responsive to the tension on drill stem 9. In some cases the effect of the friction of these components of the system may be suiiicient to have a detrimental effect on the operation of a system such as that shown in Fig. l of the drawing in which a control signal is derived from the current in the series loop. The strain gage is composed of a spring I9, pointer I9a, and resistor I9b which is connected across constant potential lines 2I and 22. In this modification, if the tension on rope 6, which is very nearly proportional to the tension on drill stem 9 and roughly proportional to the current in loop I8 increases, spring I9 is lengthened. This moves pointer I9a on resistor I9b toward connection 2I and increases the voltage drop across the section of resistor I9b, which has control field I3c of amplidyne I3 connected across it. The resulting increase in voltage supplied to field I3c causes a reduction in the current in series loop I8 in the same manner as described in the preceding paragraph. For a decrease in the length of spring I9, the apparatus functions in the opposite manner to increase the current in series loop I8. Thus, the current in series loop I8 is held at a value which maintains the strain gage I9 stationary and the tension on drill pipe 9 is maintained at the predetermined value. It will be understood that the loop current in this modification is not constant as in the previous c-ircuit but varies somewhat to compensate for the friction of the speed reducing gear and the cable and pulley system, the amount of current variation depending upon the amount of friction present in these elements. It will be further understood that our invention is not limited to the strain gage arrangement in the dead side of cable 6 as illustrated in Fig. 2, and that other means for deriving a control signal responsive to the tension on drill stem 9 for controlling the current in series loop I8 may be utilized instead. For example, a strain gage or other suitable means may be used to provide an indication of the stress in the cable or other member (not shown) which supports sheave I, or in one leg or some other portion of the tower (also not shown) from which sheave I is suspended, such signals being responsive to the tension in drill stem 9.

During the drilling operation, the excitation of machine I is maintained at a constant value. This is performed by cross armature reaction excited dynamoelectric machine I2. Excitation field winding I2c provides a constant excitation for machine I2 and the latter in turn, supplies 6 a constant voltage for the excitation of machine I through its field winding 24. Winding I2c is supplied a constant voltage derived from constant potential lines 2| and 22 through manually variable resistor 23. 1

By maintaining constant excitation and an approximately constant armature current in machine I, it is made to exert a constant tension on the drill pipe through the supporting apparatus, operating either as a drag generator or as a motor, in accordance with the needs of the system. Machine I thus supports a predetermined proportion of the weight of the drill pipe. regardless of the speed with which the bit progresses downward. This tension is maintained, even when the drill bit is stationary.

It is necessary to protect motor I against overspeeding during the drilling operation, as well as during the hoisting operation, because of the possibility of the drill bit striking a soft place in the earth and starting to go downward very rapidly. This is accomplished by comparing a signal voltage taken from tachometer 21 with a predetermined voltage derived from constant potential lines 2I and 22 through a manually variable resistor 30 and a rectifier 30a. During the time the drill pipe is moving downward the signal voltage from tachometer 21 is of opposite polarity to that indicated in Fig. 1. However, the relative polarity with respect to the constant potential lines across which resistor 30 is connected remains the same. When the signal voltage from tachometer 2! increases beyond the value of the reference voltage, derived from resistor 30, cur-v rent flows in field winding I3d in the direction which causes a decrease in the output voltage of machine I3. This decrease in the output voltage of machine I3 decreases the excitation of generator 2 if the speed of motor I becomes too great. This, in turn, increases the current circulating in series loop I8 between motor I and generator 2 above the value which is maintained during normal drilling operation. The increase in current in the series loop increases the torque output of machine I and causes it to exert more upward force on the drill pipe 9, thus reducing the rate of the downward progress of drill pipe 9 to a safe value.

With reference to resistors 20 and 29 in series loop circuit I8 from which signal voltages are derived respectively for machines I and 2 it will be understood that the signal voltages are proportional since current of equal value flows through both. A single resistor may be substituted for resistors 20 and 29 provided the elements of the control and excitation circuits are properly selected. Separate resistors are shown for purposes of clarity in explaining the invention but in practice a single resistor is usually used.

While machines I and 2 are referred to herein as dynamos in some instances, it is not to be understood from this that these machines 0perate only as generators. As is explained hereinbefore, each of these machines operates both as a generator and a motor during a complete cycle of operation of the control system. Machines I and 2 are referred to as dynamosin order to distinguish them readily from dynamoe electric machines I2 and I3. Machines I and2 would be described more accurately perhaps by the apellation dynamoelectric machines and they would have been so designated except for the possibility of confusion with machines I2 and I3.

While we have illustrated and described one embodiment of our invention together with one modification, many additional modifications will occur *to those skilled in the art and,therefore, it should be understood that'we intend to'cover, by the appended claims, all such modifications as fall within the'true spirit and-scope of this invention.

What We'claim as new and desire to secure by Letters Patent of the United- States is:

l. A control system for rotary oil well drilling rigs and the like comprising in combination, an adjustable voltage direct current electric dynamo having a prime mover and power absorption device-coupled-thereto, a second reversible-direct current-electric dynamo having coupled thereto a mechanism to be drivenand controlled, said mechanism having a controllable condition, the two said dynamos having their armatures connected in aseries loop circuit, strain gage means for deriving a control voltage responsive to said controllable condition, means for selectively maintaining a constant torque on the output shaft of said second reversible dynamo regardless of the rotation of said second dynamo whereby said controllable condition is maintained at an 1 approximately constant predetermined value, said last-named means comprising means for maintaining the field current of said second dynamo at a constant value and simultaneously maintaining the current in said series loop circuit at an approximately constant predetermined value and including a cross armature reaction excited dynamoelectric machine having a control field Winding connected to be excited responsively to said control voltage, means for q derivinga second control voltage proportional to the speed of said second dynamo and'means for limiting the speed of said second dynamo comprisingan additional field winding on'said dynamoelectric machine connected to be excited responsively to said second control voltage.

2. A control system for rotary oil well drilling rigs and the like comprising in combination, an adjustable voltage direct current electric-dynamo having a prime mover coupled thereto, a second reversible direct current electric dynamo coupled to a mechanism to be driven and controlled, the armatures of the two said dynamos being connected in a series loop circuit, a resistor n said loop circuit from which a control voltage proportional to the current flowing in said loop circuit is obtained by utilizin the voltagedrop across said resistor, means providing an adjustable reference voltage comprising a constant voltage source and a manually variable resistor, con nected thereto, means for maintaining a constant horsepower output from said second dynamo, said'means comprising a cross armature reaction excited dynamoelectric machine connected to be excited by a voltage equal to the sumof said reference voltage an'd said control voltage, means 'for derivinga second control voltage proportional to the speed of said second dynamo,

and means for limiting the speed of said secvoltage :electric dynamo connected; to :a mecha-'- nism to :bel-driven and controlled and having its armature connected in a series loop circuit with the armature :of said firstdynamo, .means for derivingamontrol voltage responsiveto the current flowingin 'saidloop circuit, selective means responsive toisaidcontrol voltage for controlling the excitation of said second dynamo'to operate it :as a motor and maintain a substantially constant horsepower output, means for deriving a secondcontrol voltage responsive to a-controllable :condition of said mechanism, and selective means responsive tosaid second control voltage for controlling the excitation of said first dynamo to maintain. saidconditionat a predetermined value.

4. A control system (comprising afirst adjustable voltageelectric dynamo, a second adjustable voltage electric .dynamo connected to a mechanism.to be driven'and-controlled and havingits armatureiconnected in a series loop circuit with the armature of said'first dynamo, said'mechanism having a controllable condition, means includingha resistor connected in said series circuit loopior deriving a control voltage proportional to the current 'ilowing in said loop circuit, .a source of reference voltage, selective means :responsive tothe sum of said control voltage and saidlreference voltageforcontrolling the excitation of said second dynamo to operate it as .a motor and provide .aisubstantiallyfl constant;horsep.ow.eri output for the operation ofv said mechanism over aconsiderable range of speeds, means'ior deriving a second control voltage proportional. to the current. flowing in said loop .circuit, a secondsourceof reference voltage, and selective means-responsive tothe sum oi said second control voltage and said second reference voltage for controlling the excitation of saidiirst dynamo to maintain saidicondition at a predetermined value.

5.. 'A controlv system J comprising. a first adjustable voltage electric dynamo, a second adjustable voltage electric .dynamo connected to a hoist mechanism to beldrivenwand controlled and-having its iarmatureconnected. in a series loop circuit withthe armature .of said first-dynamo, means forderivinga control voltage responsive to;.the;current flowing in said loop circuit,selectivetmeans responsive to said control-voltageior controllingpthe excitation of said second dynamo toioperate'it as aimotor .to raise-the hoist and provide a substantially constant horsepower output to'ssaid hoist mechanism regardless of hoisting speed, strain-gage means for-deriving a-second control voltage responsive to the tension on the hoistingcable of said hoisting mechanism, and selective means responsive to said second control -volta ge for controlling the excitation of said first'dynamo tomaintain said tension'at a predetermined valueregardless'of the speed of saidhoistingmechanism.

6. A'control systemcomprising a first adjustable vo-ltageelectric dynamo adaptablefor operationiprimarily as a motor, a second electric dynamo adaptable for operation ,primarilylas .a drag Efinerator connected to. a lowering mechanism and having its armature connected in a series loop circuit with the armature of said first dynamo, said mechanism being provided with a load supporting cable, means for deriving-a control voltage responsive to the tension in "said cable, and means responsive'to said control volt age for controlling theexcitation of said :first dynamo to maintain said tension at a selected value.

7. A first adjustable Voltage electric dynamo adaptable for operation primarily as a motor, apower absorption device associated with said first dynamo, a second adjustable electric dynamo adaptable for operation primarily as a generator connected to a lowering mechanism and having its armature connected in a series loop circuit with the armature of said first dynamo, said lowering mechanism including a load supporting cable, strain gage means for deriving a control voltage responsive to the tension in said cable, means responsive to said control voltage for varying the excitation of said first dynamo to maintain said tension at a selected value, means for deriving an additional control voltage responsive to the speed of said second dynamo, and means responsive to said additional control voltage for varying the excitation of said first dynamo to limit the downward speed of said lowering mechanism.

8. A control system comprising a first adjustable voltage electric dynamo, a second electric dynamo connected to a hoisting and lowering mechanism and having its armature connected in a series 100p circuit with the armature of said first dynamo, said hoist mechanism including a cable for raising and lowering a load, means for deriving a control voltage responsive to the tension in said cable, said means comprising a resistor in said series loop circuit, and means responsive to said control voltage for controlling the excitation of said first dynamo to maintain said tension at a selected value during lowering regardless of the speed of said hoist, and means for limiting the speed of said second dynamo comprising means for deriving an additional signal voltage responsive to the speed of said second dynamo and means responsive to said additional control voltage for affecting the excitation of said first dynamo.

9. A control system for rotary drilling rigs and the like comprising in combination, a first adjustable voltage direct current electric dynamo having a prime mover and power absorption device coupled thereto, a second reversible direct current electric dynamo having coupled thereto a mechanism to be driven and controlled, said mechanism having a controllable condition, said two dynamos having their armatures connected in a series loop circuit, a resistor connected in said loop circuit for deriving a control voltage proportional to the current flowing in said loop circuit, a reference voltage derived from a constant voltage source, means for selectively securing an approximately constant horsepower output from said second dynamo, said means comprising an amplidyne exciter connected to be excited by a voltage equal to the sum of said reference voltage and said control voltage, means for deriving a second control voltage responsive to the controllable condition of said mechanism, and selective means for regulating said system to maintain said controllable condition at a predetermined value, said means comprising means for maintaining the field current of said second dynamo approximately constant and amplidyne exciter means for said first dynamo responsive to said second control voltage,

10. A control system for rotary oil well drilling rigs and the like comprising in combination, an adjustable voltage direct current electric dynamo having a prime mover and power absorption device coupled thereto, a second reversible direct current electric dynamo to which is coupled a mechanism which is to be driven and controlled, the two said dynamos having their armatures connected in a series loop circuit, a first resistor in said loop circuit from which a first control voltage proportional to the current fiowing in said loop circuit is obtained by utilizing the voltage drop across said resistor, a first reference voltage derived from a constant voltage source through a first variable resistor, means for selectively maintaining a constant torque on the output shaft of said second reversible dynamo regardless of the rotation of said second dynamo comprising means for maintaining the field current of said second dynamo constant and means for maintaining the current in said series loop circuit constant, said last-named means comprising a cross armature reaction excited dynamoelectric machine having a control field winding connected to be excited by a voltage equal to the sum of said first reference voltage and said first control voltage, a second resistor in said loop circuit from which a second control voltage proportional to the current flowing in said loop circuit is obtained, a second reference voltage derived Irom said constant voltage source through a second variable resistor, means for selectively securing a constant horsepower output, from said second dynamo comprising a second cross-armature reaction excited dynamoelectric machine connected to be excited by a voltage equal to the sum of said second reference voltage and said second control voltage, means for deriving a third control voltage proportional to the speed of said second dynamo, means for limiting the speed of said second dynamo comprising an additional field winding on said first dynamoelectric machine connected to be excited by the difierence between said third control voltage and a third reference voltage derived from said constant voltage source, and rectifier means in circuit with said additional field winding permitting current to flow in one direction only through said additional field winding.

11. A control system comprising a first adjustable voltage electric dynamo adaptable for operation primarily as a motor, a second electric dynamo adaptable for operation primarily as a drag generator connected to a lowering mechanism and having its armature connected in a series loop circuit with the armature of said first dynamo, said mechanism being provided with a load supporting cable, means for deriving a control voltage responsive to the tension in said cable, means responsive to said control voltage for controlling the excitation of said first dynamo to maintain said tension at a selected value, and means for limiting the speed of said second dynamo to a predetermined value.

MARTIN A. EDWARDS. ROBERT A. BRAND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,174,772 Steiner Mar. 7, 1916 2,285,654 Hanna et al. June 9, 1942 2,412,888 Ivy Dec. 17, 1946 2,469,899 Sills May 10, 1949 2,473,721 Montgomery et a1. June 21, 1949 2,508,154 Frisch May 16, 1950 

